One method of recovering the uranium values present in phosphate fertilizer deposits involves the oxidation of the uranium values that are present in wet process phosphoric acid streams and the extraction of the oxidized uranium from the acid into an immiscible organic extractant. Organic extractants which effectively extract the oxidized uranium (U.sup.+6) are known in the art and contain a combination of dialkyl phosphoric acid and trialkyl phosphor oxide. An example of such an extractant is a liquid hydrocarbon diluent containing di(2-ethylhexyl) phosphoric acid (D2EHPA) and trioctylphosphine oxide (TOPO). One such process for uranium recovery from wet process phosphoric acid (WPA) is reported in detail in ORNL-TM-2522, (a U.S. A.E.C. report) entitled "Solvent Extraction of Uranium from Wet-Process Acid" by F. J. Hurst et al.
An effective method of subsequently stripping the uranium from the organic extractant involves the use of an aqueous phosphoric acid solution containing ferrous ions. The aqueous raffinate (a phosphoric acid solution) from the first cycle is a suitable stripping solution provided it contains ferrous ion to effect the reduction of the uranium U+6 to U.sup.+4 during the stripping operation, i.e., during the mixing of the aqueous strip and pregnant organic phases. An effective such reductive stripping operation is reported in detail in ORNL TM-4572 a U.S. A.E.C. Chemical Technology Div. Progress Report at pages 185 and 186 (October 1970).
Various oxidzing agents may be considered to first oxidize the uranium to the +6 state uranyl ion (UO.sub.2 +2) with various attendant advantages and disadvantages. Nitric acid is considered to be a particularly suitable oxidant. Relatively inexpensive, it will rapidly oxidize the wet process acid (which may be indicated by measuring the Redox potential) to provide rapid efficient extraction into the organic containing D2EHPA-TOPO. Theoretical explanations of nitric acid oxidation reactions in general suggest that oxidation may proceed because of the presence and generation of nitrite ion (NO.sub.2.sup.-). A process incorporating the use of nitric acid and nitrite to oxidize wet process acid prior to first cycle extraction is considered to be generally advantageous.
We have found that the organic phase which extracts the oxidized uranium also has an affinity for and therefore extracts both nitrates and nitrites that may persist as contaminants in the nitric acid oxidized wet process acid. We have also found that the presence of these nitrates or nitrites in the extractant reduces the effectiveness of the subsequent reductive stripping operation. Apparently, the nitrates and/or nitrites react with the ferrous ion in the aqueous strip solution. The effectiveness of the reductive stripping operation is reduced because of the apparent reduced availability of ferrous ion to reduce the oxidized uranium during the stripping operation. The ferrous ion acts as a driving force in the stripping operation. Increasing the concentration of the ferrous ion by the increment that reacts with the contaminating nitrate-nitrite ions is a way of restoring the efficacy of the stripping operation but it also causes the ultimate uranium product to be more highly contaminated with the added excess iron. Maintaining the effectiveness or efficiency of the reductive strip without the subsequent iron contamination would, of course, be desirable.